Shutter leaf control system



- Nov. 18, 1969 I D. A. JONES 3,478,664

SHUTTER LEAF CONTROL SYSTEM i Filed July 31, 1967 v 3 /4 INVENTOR United States Patent 3,478,664 SHUTTER LEAF CONTROL SYSTEM Derrick A. Jones, Woodbury Township, Minn.,

to Minnesota Mining and Manufacturing Company, Paul, Minn., a corporation of Delaware Filed July 31, 1967, Ser. No. 657,130 Int. Cl. G03b 9/10 assignor St.

US. Cl. 95-59 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of invention This invention relates generally to photographic shutters of the type having leaves or blades which move or pivot to cover or uncover a lens aperture. More particularly, the invention relates to a system for restraining the movement of shutter leaves, and in one aspect to latching and braking shutter leaves in cameras used, for example, in aerial photography which have long focal length lenses with wide apertures.

Description of the prior art One common type of shutter mechanism uses two separate but concentric and parallel opposing multi-leaf systems. One of the two multi-leaf systems, initially in closed position, is first opened to uncover the lens aperture and then, after an appropriate time interval, the second and initially open multi-leaf system is closed to again cover the aperture.

The leaves of the above first mentioned system are biased or powered toward the open position and, prior to exposure actuation, are cocked or loaded against the bias and latched in the closed position. Actuation then consists of releasing the latched leaves which are then driven to the open position where they are stopped or braked. And similarly, the leaves of the above second mentioned system are biased toward the closed position and are cocked in the open position. After an appropriate time interval following actuation of the first system, the second system is actuated by releasing the leaves which then are powered toward the closed position where they are braked.

One method which has been suggested for controlling shutter leaves is to latch, release, and brake the leaves magnetically. For example, in Larson, US. Pat. No. 2,953,983, each pivotally mounted leaf has a rigid armature of iron or steel which is attracted or restrained by a magnetic flux produced by appropriately positioned electromagnets, i.e., the leaves are magnetically restrained in a cocked position by one electromagnet, released therefrom, and magnetically braked in their final position by another electromagnet.

Two inherent problems of an electromagnetic shutter control are (1) the need for iron or steel or other massive magnetic material to act as the armature on the leaves, and (2) the time required to build up and remove the magnetic flux or field. Materials having the required magnetic properties, used as the armature on the leaf, are generally relatively heavy and consequently increase leaf inertia,

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which adversely effects both shutter speed and braking. And although the time required to build up or remove a magnetic field is relatively short, e.g., about one millisecond, it does impose a restriction on response time, i.e., the time between trigger pulse and shutter movement.

SUMMARY OF THE INVENTION The present invention comprises generally a system for controlling or restraining the movement of shutter leaves electrostatically, or, more specifically, by utilizing the Johnson-Rahbek effect. This is accomplished generally by interposing a resistive layer between a conductive surface ona leaf and a conductive surface on a stationary restraining member, one side of the resistive layer being secured to one of the conductive surfaces, the other side of the layer having a surface in frictional point contact with the other conductive surface, and applying an electrical potential between the conductive surfaces so as 1: create an electrostatic attraction therebetween.

The leaves can be formed of a low mass material and are electrically conductive over at least a portion of their surface. The restraining member or pad is fixed or stationary with respect to the movable leaves and has an electrically conductive surface facing, parallel to, and in close proximity to the conductive leaf surface. In one embodiment, the resistive layer is formed of a semiconductive material and has one side secured to the conductive restraining member surface and has the other side in actual contact with the conductive surface of the leaf. The actual contact between the semiconductor surface and the conductive leaf surface occurs only at points of surface roughness. Even when the surfaces are highly polished, the area of contact relative to the total area of the surfaces is only about one percent. Consequently, except for the point contacts, the leaf surface and semiconductive surface are spaced apart a finite distance and have an air gap, or other insulation material, interposed therebetween.

Upon application of an electrical potential between the conductive leaf surface and the conductive surface of the restraining member an electrostatic attraction is developed between the two which, through the points of actual contact, produces a frictional restraining effect on the movable leaf.

In operation the potential difference is applied to latch the leaves in a cocked position, removed to release the biased or powered leaves, and again applied to brake or stop the leaves at their expended or closed position.

In other embodiments, the resistive layer or semi-conductor may be secured on one side to the conductive leaf surface and slideably frictionally engage by point contact the conductive surface of the restraining member. Likewise, either the total leaf surface or only a portion thereof may be made electrically conductive.

The shutter leaf control mechanism of the present invention permits low inertia leaf construction and extremely fast response time. Electrostatic attraction is a surface phenomenon and consequently the application of force to the shutter leaf may be spread over a large area without adding to the inertia of the leaf. Furthermore, the response time of the system of the present invention is approximately one or two microseconds as opposed to about one millisecond for electromagnetically controlled shutters.

DESCRIPTION OF DRAWING FIGURE 1 is a fragmentary plan view showing one leaf of a five leaf shutter;

FIGURE 2 is an elevation of shutter, partially in section, taken along line 22 of FIG. 1, including a diagramatic circuit;

FIGURE 3 is a cross-sectional view taken along the section line 3--3 of FIG. 1; and

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 illustrates one leaf and leaf control mechanism of a five leaf shutter, the other leaves and control mechanisms being identical but left out for purposes of clarity. The leaf system of the present invention, one segment of which is'shown in the drawing, is interposed within the housing of a camera between the lens system and the light sensitive film.

System comprises generally a circular insulating base plate 11 having an aperture 12 which is either completely covered or uncovered by operation of movable leaves 13. Each leaf 13 pivots about pin 14 between an open position and,as shown by dotted lines in FIG. 1, closed position, and is biased or powered toward one or the other of such positions. In FIG. 1 the leaf 13 is biased toward a closed position by spring 16 which at one end engages eyelet 17 of leaf arm 18 and at the other end is anchored to screw 19.

Bore 21 in base 11 rotatably receives a cylindrical stem portion centrally depending from lower restraining member 22 formed of a conductive metal and having a flat surface 23 to which a thin semiconductive disc or pad 24 is bonded by a conductive cement.

Extending upwardly from base 11 is upright support 26 having a bore 27 substantially in alignment with bore 21. Restraining member 28, formed of a conductive metal, has an indentation which rotatably receives a ball point of spring housing 31, which is rotatably housed within bore 27. Compression spring 32 presses at its lower end against the floor of spring housing 31 and at its upper end against contact bar 33, which is held in place upon upright 26 by screws 34. Restraining member 28, which has a flat surface 29 to which semiconductive pad or disc 30 is conductively bonded, is forced downwardly toward the upper leaf surface 37 by the action of spring 32.

semiconductive discs 24 and 30 are in sliding frictional contact with the lower and upper surfaces 36 and 37, respectively, of leaf 13. FIG. 4 illustrates a small portion, in enlarged scale, of the interface of the semiconductive pads 24 and 30 and leaf surfaces 36 and 37, showing that even when highly polished actual contact exists only at but a few points.

A circuit for establishing the potential difference between the conductive surfaces 36 and 37 of leaf 13 and the conductive surfaces 23 and 29 of restraining members 22 and 28 is illustrated in FIGURE 2. One portion of the circuit includes two separate branches including conductors 41 and 42. Conductors 41 and 42 are electrically connected to one terminal of a voltage source 38 through a contact 43 of a timer or intervalometer 37. Lead 41 is electrically connected to restraining member 22 and applies a potential between conductive surface 23 and surface36 of leaf 13. Lead 42 applies the same potential via contact bar 33, conductive spring 32, and spring housing 31 to the upper restraining member 28. The flat conductive surface 29 is at the same potential as upper restraining member 28.

The other portion of the electrical circuit includes a lead 44 which is electrically connected directly to the other terminal of voltage source 38. Lead 44 is electrically connected via screw 19 and spring 16 to conductive leaf 13. In this manner leads 41 and 42 apply one terminal of the voltage source 38 to conductive surfaces 23 and 29. Similarly, the other terminal of voltage source 38 is connected to leaf 13. When the contact 43 is selectively closed, the circuit is energized thereby applying an electrical potential between the two sets of conductive surfaces, the first set being 23 and 36 and the second set being 29 and 37,

In operation the leaves 13 are cocked, utilizing known cocking apparatus such as, for example, that disclosed in the before-mentioned Larson patent, to a position against the bias, shown by the solid line in FIG. 1. At the illustrated open position, the circuit is closed producing a potential difference between the surfaces of the leaf and the surfaces of the restraining members creating an electrostatic attraction between the two alone sufiicient to hold the leaf open against the spring bias. When film exposure is desired, the switch or intervalometer is actuated which precisely momentarily opens and then closes the voltage supply circuit. When the circuit is'opened the potential difference is removed, and consequently the electrostatic attraction is also removed, and the leaf is powered by the spring toward the closed position shown by dotted lines in FIG. 1. The circuit is then closed to again apply a potential difference and electrostatic attraction to stop the leaf at the closed position.

In the illustrated embodiment the leaf is formed of a light-weight, high-strength, conductive material such as titanium. In other embodiments the surfaces of the leaf over at least a portion of the area defined by the stationary pads upon the leaves moving between the open and closed positions are electrically conductive, and have a conductive lead to complete the circuit, while the remainder of the leaf is formed of any suitable low mass material.

Exemplary of parameters useful in the shutter control system of the present invention, the semiconductive or resistive pads have a resistivity within the range of about 10 to 10 ohm cm., the spring urges the pads against the leaf with a force of about 40 gms. per cm. and the potential difference between the pads and leaves is within the range of about 300 to 400 volts. The semiconductive pad may be formed of known materials, such as, for example, the partially carbonized polymeric material disclosed in British Pat. No. 994,676.

As an example of the attractiveforces achieved between the leaf and restraining member, when a potential of 300 volts is applied a force of about 5,000 gms. per cm? is produced. The forces produced are generally a function of the voltage and the effective width or distance of the air gap. Variations in the factors determining the force produced can be made by one skilled in the art to suit the particular application desired.

conventionally, the present shutter system is interposed between the lens system and film within the housing of a camera, but other variations may be made by those skilled in the art as, for example, integrating the shutter system of the present invention within a lens system. Additionally, the speed of the shutters, the weight of the leaves, etc., can be varied to make the system suitable to various different photographic applications.

What is claimed is:

1. In a photographic shutter of the type having at least one leaf movable between a first position and a second position, bias means for moving said leaf from said first position toward said second position, cocking means for moving the leaf from said second position to said first position, and restraining means for restraining the move ment of said leaf; the improvement of said restraining means which comprises:

(a) a restraining member fixed in relation to said movable leaf and having a surface in proximity to and adjacent a surface of said leaf,

(b) said leaf and said restraining member being adapted to maintain a parallel relationship between their said surfaces in the direction of movement of said leaf,

(c) at least a portion of each of said surfaces being in face-to-face relationship with the other and being electrically conductive,

(d) a resistive layer having greater transverse resistance than either of said conductive surfaces positioned between the conductive surfaces,

(c) said layer having one surface having points in frictional contact with one of said conductive surfaces and the other surface of said layer being secured to the other of said conductive surfaces and in ohmic contact therewith,

(f) means for applying an electrical potential between said conductive surfaces sufficient to electrostatically attract said surfacs toward each other whereby the movement of said leaf is restrained.

2. In a photographic shutter of the type having at least one leaf movable between a first position and a second position, bias means for moving said leaf from said first position toward said second position, cocking means for moving the leaf from said second position to said first position, and restraining means forrestraining the movement of said leaf; the improvement of said restraining means which comprises:

(a) at least one pair of restraining members, one member on each side of each of said movable leafs and in fixed relation thereto, each 'member having a surface in proximity to and adjacent a surface of said leaf;

(b) said leaf and said restraining members being adapted to maintain a parallel relationship between their said surfaces in the direction of movement of said leaf; (c) said surfaces being electrically conductive;

(d) a resistive layer having greater transverse resistance than either of said conductive surfaces positioned between the conductive surfaces;

(e) said layer having one surface having points in frictional contact with the surface of said leaf and the other surface of said layer being secured to the surface of said restraining member and in ohmic contact therewith; and

(f) means for selectively applying an electrical potential between said conductive surfaces suflicient to electrostatically attract said surfaces toward each other whereby the movement of said leaf is restrained.

References Cited UNITED STATES PATENTS 2,856,831 10/1958 Gipe -53 3,055,281 9/ 1962 Anderson 95-59 3,348,460 10/ 1967 Schmitt 95S3 JOHN M. HORAN, Primary Examiner LEO H. MCCORMICK, IR., Assistant Examiner US. Cl. X.R. 9553 

